High Intensity Discharge (“HID”) lamps, being among the most efficient light sources available at up to 100 lumens per Watt, are frequently used for large area illuminations. For example, the number of installed HID units worldwide exceeds 400 million, and by one estimate, approximately eight percent of the world's electricity production is used in HID lighting. Additionally, more than half of these installed 400 million units are used for roadway and parking area lighting, and approximately half of these installed 400 million units make use of either 250 W or 400 W lamps.
However, due to the negative resistance characteristic of HID lamps, electrical ballasts are required to ignite and thereafter limit the flow of current to the HID lamp. Thus, the electrical ballast device has three primary functions: (1) lamp ignition, (2) lamp power control, and (3) control of AC line transients and power quality.
Traditionally, the above tasks have been accomplished through the use of magnetic ballasts similar to those utilized in fluorescent lighting that incorporate metal coil transformers. However, magnetic ballasts have several inherent deficiencies. For example, in addition to being physically heavy, magnetic ballasts frequently produce a magnetic humming noise, are inefficient at converting input power to proper lamp power, and have less-than-ideal “power factors,” thereby creating difficulties for local power companies. Moreover, magnetic ballasts are typically not dimmable, must be replaced every five to ten years, and power line variation can impact the output of the magnetic ballast, thereby creating fluctuations in the output of the connected HID lamp. Accordingly, recent efforts to improve HID lighting have led to the introduction and promotion of electronic arc lamp ballasts as an alternative to the older magnetic ballasts.
FIG. 1 provides a circuit diagram of a typical electronic are lamp ballast known to those skilled in the art, and FIG. 2 provides a perspective view of the housing of such a typical electronic arc lamp ballast. Compared to magnetic ballasts, electronic ballasts are lighter (up to 11 pounds), more efficient (up to 30%), quieter, have longer service lives, and have a nearly-perfect “power factor” (99% compared to an upper limit of approximately 90% for magnetic ballasts). However, the widespread adoption of electronic ballasts has been tempered, at least in part, by the difficulty associated with retrofitting existing light fixtures designed for magnetic ballasts with electronic ballasts. In particular, currently-available electronic ballasts for HID lamps are often simply too large to fit within the housings of many HID light fixtures. Additionally, currently-available electronic ballasts for HID lamps are not configured to dissipate heat through the housing of the HID light fixture as are pre-existing magnetic ballasts, thereby creating over-heating problems within the electronic ballast even if it were able to fit within the housing of the HID light fixture. Thus, to retrofit HID light fixtures using pre-existing magnetic ballasts with newer electronic ballasts, significant infrastructure modifications are often required, thereby drastically increasing the cost of such a project and making it much more difficult to recover from an upgrade to electronic ballasts should the decision to do so later be reversed.